1 The purpose of this project is to reconstruct Gilbert Foliot's view of consuetudo by analyzing his letters. He was bishop of London from 1163 to 1187 and acted as a papal judge delegate and as a royal judge as well. Therefore, to reconstruct his legal view seems to contribute toward the understanding of the role of the learned laws (Roman and Canon Law) in the early history of the Common Law.2 The main research results are summarized in the following points.(1) His early letters which were written while he was still bishop of Hereford sometimes show that he was much interested in the learned laws. But, from the point of making clear his view of consuetudo, his letters written after the Constitutions of Clarendon in 1164 are more important. In particular, the letter "Multiplicem nobis" addressed to Thomas Becket in late 1166 is most important.(2) In that voluminous letter, he, while criticizing Thomas, insists that consuetudo embodied in that Constitutions was not created by king Henry II,and that the cooperation between regnum and sacerdotium has been and is essential to the maintenance of peace. This rather traditional view is seen in his argument about temporalia and spiritualia as well.(3) Judging from that letter, we should reconsider the accepted view that the church in defending the libertas ecclesie opposed to consuetudoas a legal source.